This time, after a week of air strikes, Israel believes force will work, because much more of it is being used.

The Israeli military will be concerned about casualties, but they are a very formidable and well-armed, modern force.

Its troops also know Gaza very well, having occupied the coastal territory from 1967 until September 2005.

Nevertheless, they may find it difficult when they enter the main populated areas, in which the buildings stand pretty close together. The many refugee camps are warrens of single-storey, concrete or breeze-block houses with tin roofs.

Bravado

Hamas fighters, who know Gaza better than the Israeli troops, have a belief in resistance and martyrdom.

Their objective will be to give the Israeli army the same humiliation that Hezbollah inflicted during 2006.

For all their bravado, Hamas are unlikely to be as formidable as Hezbollah.

The rocky, hilly terrain of south Lebanon was not good for Israel's armour. Gaza, on the other hand, is flat and sandy.

Hamas is said to have smuggled weapons in through tunnels under the border Egypt, but it is doubtful that it has the arsenal which Hezbollah acquired.

Even so, they will be the latest heroes to those people across the Islamic world who have latched onto ideology of resistance to Israel and its American allies, which has become one of the region's most potent ideas.

For Hamas, the definition of victory will be that they can they fight on at the end of all this; for Israel it will be the stopping of rocket fire from Gaza.

Decision of arms not diplomacy in Gaza

Diplomacy will have to wait until there is a decision of arms in Gaza.

At the moment, diplomacy carries little weight

This conclusion is inevitable in the face of a determination by the Israeli government that it faces an unacceptable threat from Gaza that must be dealt with.

By delaying ground operations for a week, it gave Hamas a chance to back down and call a halt to the firing of rockets into Israel. But Hamas chose confrontation, probably fearing that to do otherwise would be to show weakness.

Theoretical solution

In theory, a diplomatic solution is on the table. It would consist of the following: a mutual ceasefire, a prohibition on the smuggling of arms into Gaza and a relaxation of the Israeli economic squeeze on Gaza.

This would give something to each side. There would be calm across the border, Israel would get some assurance - which would have to be backed up by an enforcement mechanism - of reducing at least the entry of arms into Gaza and Gazans would see the prospect of an improvement in their daily lives.

However when or even if it is possible to talk seriously of such an agreement remains to be seen. On Saturday evening, after the Israeli ground operations began, the Security Council found itself unable to agree on a new call for a ceasefire.

The United States blocked a statement with the argument that Hamas had not agreed to a previous ceasefire call. A new statement, in the words of the US deputy ambassador Alejandro Wolff, "would not be adhered to and would have no underpinning for success, [and] would not do credit to the council".

European visitors

The visit of President Sarkozy of France, who arrives in Egypt on Monday and then goes on to Israel and the Palestinian territories, will lead to more talk of a cessation and the conditions attached to it.

The former UK Prime Minister Tony Blair, who is the representative of the Quartet of the US, the UN, Russia and the EU, has also been busy.

The European Union's troika of foreign ministers is also visiting on Monday, but at this stage of a major conflict like this, outsiders from Europe cannot make an effective intervention.

With the Bush administration blaming Hamas, Israel has the diplomatic support it needs.

Not that Israel will want to ignore international opinion. It has been careful in its operations to stress that it will maintain humanitarian supplies into Gaza.

What outcome?

The unanswered question is whether Israel will succeed, first in its military operations to stop the rockets, and then in its diplomatic aim of getting a durable arrangement.

The war in Lebanon showed the difficulties of combat against a well-entrenched militia-type organisation.

So there is a danger for Israel of its troops getting bogged down to no great effect.

Much depends on whether Hamas has been able to recover from the shock it must have received eight days ago when the air assault began.

On the other hand, a decisive Israeli ground intervention could leave the door open to negotiation - but only if Hamas chooses to walk through that door.

Hamas ready for bitter urban battle

Israel's move to ground operations in the Gaza Strip gives Palestinian militants their first chance to trade blows on a more or less equal footing with the Israeli army in the current conflict.

Until now the militants have been impotent to counter Israel's air, sea and ground bombardment - spheres where it enjoys total military superiority.

Hamas and other militant groups have launched dozens of rockets at Israel but - while these are occasionally lethal and cause considerable disruption and sap civilian morale - they are ineffective militarily.

Soldiers of the enemy... you must know that a black destiny is waiting for you

Hamas leader Khaled Meshal

But if the conflict transfers to the narrow alleyways of Jabaliya camp, or any of Gaza's teeming urban areas, it will be a different story.

"Hamas has few tools compared with the Israeli army," says Nicolas Pelham, Senior Analyst with the Middle East Programme of the International Crisis Group.

"But the Israeli operations in built-up urban areas will eliminate some of that huge difference," he added.

Israel possesses formidable tank forces, armoured bulldozers, uncontested use of air power and all the paraphernalia of a modern army, such as night vision and thermal imaging equipment.

But Hamas has had months to prepare for bitter urban warfare which will give its fighters a chance to inflict casualties on the Israeli military.

Smuggled capability

Hamas is by nature a secretive organisation.

Israeli troops spent 38 years occupying Gaza, but withdrew in 2005

However, analysts believe it has considerably enhanced its military capability since taking control of the Gaza Strip in the summer of 2007.

The takeover handed Hamas the limited arsenal of the routed Palestinian Authority. But more importantly it gave Hamas freedom to operate throughout Gaza without interference from the PA, which was committed to disarming militant groups.

Hamas has had to smuggle all its other weaponry into Gaza - which is under a complete blockade imposed by Israel and supported by Egypt in the south.

This is done via tunnels under the Egypt-Gaza border, which have furnished Hamas with the medium-range rockets that have hit Israeli cities up to 25 miles (40km) away.

Explosives and the ingredients for explosive and rocket propellants are even more easily smuggled into Gaza.

Mortars, anti-tank weapons and some anti-aircraft weapons - not effective against modern jet fighters, but possibly so against older helicopters - are also thought to have come through the Rafah tunnels.

So it was an unsurprisingly bellicose Khaled Meshal, Hamas's Damascus-based leader-in-exile, who spoke in an Arabic TV interview before ground operations began on Saturday night.

"Soldiers of the enemy... you must know that a black destiny is waiting for you, and you will either be killed, injured or imprisoned," he told al-Jazeera television.

Lessons of 2006

The Hamas military wing - the Izz al-Din Qassam Brigades - is thought to have about 15,000 members.

It is therefore vastly outnumbered by Israel's total military capability. But there is no shortage of weapons in Gaza or people who might want to pick up a gun against Israelis forces.

Standards of training and discipline for the Brigades have been raised recently, and it is believed to have fairly sophisticated communications systems.

Palestinian civilian casualties are likely to rise in any ground engagement

The organisation has clearly learnt lessons from the 2006 Lebanon war, when Hezbollah bloodied Israel's nose in a bruising encounter in terrain ideal for guerrilla warfare against a conventional army.

Since then, a number of Hamas members are thought to have spent time with Hezbollah and Iran's Revolutionary Guards movement - leading to improvements in its military training regime and organisational system.

The geography of Gaza may not be as advantageous as south Lebanon, but doubtless many Izz al-Din Qassam Brigades members will be hoping to deal a blow against the Israeli army and be rewarded with what they consider glorious martyrdom in the process.

Israel too, however, appears to have made changes since 2006, analysts say.

Its forces have trained hard for just such an engagement, as well as working on improvements to civil defence, supply lines, planning and public relations.

But the important question now is what does Israel want to achieve from ground operations. The stated Israel aim is to deal a blow to Hamas and prevent rocket fire.

But if the army does seek to retake urban areas it could play directly into the hands of Hamas - especially the longer its heavy armour remains there. And what happens when Israel pulls out?

"There's no guarantee even if Hamas is removed from power in Gaza that rocket fire will not continue," says Nicolas Pelham of the ICG.

"The fact is that more rockets were being fired from Gaza before the Hamas takeover, not least in the era of chaos, than afterwards."

What seems certain is that Hamas' desire to fight in the heavily populated urban areas, and Israel's apparent willingness to prosecute its war there, could have a devastating affect on Gaza's long-suffering civilian population who have nowhere else to go.

During the day, the fighting appeared to move away from the northern end of the territory, towards more populous areas in the west, correspondents say.

Later, Israeli military sources and witnesses said Israeli tanks and heavy armour had taken up positions on either side of Gaza City, in effect cutting Gaza into two parts, from the Karni crossing to the Mediterranean Sea.

The town of Beit Hanoun was also reportedly surrounded.

Hamas officials and witnesses report major fighting in five areas: east of the Jabaliya refugee camp; in the Zeitoun area; near the site of the former Jewish settlement of Netzarim; in the centre of Gaza; and on the outskirts of Khan Younis.

Hamas said its fighters were in some cases engaged in "face-to-face battles" with Israeli soldiers.

Earlier, the Israeli military said the militants were not engaging its troops in close combat but using mortars and improvised bombs.

The Palestinian health ministry says 509 Palestinians, mostly civilians, have been killed since the Israelis began their assault on Gaza eight days ago.

It says 21 of the 70 people killed since the beginning of the ground offensive were children. Some 2,500 people have reportedly also been wounded.

The figures could not be independently verified. Israel is refusing to let international journalists into Gaza despite a ruling by its a supreme court to admit a limited number of reporters.

Hamas officials say that 10 of its fighters have so far been killed.

The Israeli military says one of its soldiers has been killed and 34 wounded in the ground offensive, three of them seriously. It believes about 80% of the Palestinians killed were Hamas members.

'Trickle of aid'

The BBC's Rushdi Abu Alouf in Gaza City says the fighting and Israeli positions have stopped desperately needed medical supplies getting through to hospitals that are struggling to cope with the casualties.

The aid agency Oxfam said a paramedic working for a partner organisation had been killed and two others injured by an Israeli shell.

Oxfam said it had been forced to suspend its work, apart from emergency medical aid, because the "trickle of humanitarian aid that Israel has sometimes allowed in" had dried up since the start of the ground offensive.

The Israeli government says 400 truckloads of humanitarian aid has been allowed into Gaza since the operation began.

Many agencies say deliveries have been insufficient, and that it is difficult to get supplies to where they are most needed.

The main aid crossing point at Rafah has also been closed while Israel troops attempt to destroy tunnels under the border with Egypt.

Map: Gaza and Israel conflict

Israeli forces are conducting an offensive against Hamas in Gaza by land, sea and air, while Palestinian rockets continue to hit Israel. This map details the latest major developments.

Numbers refer to locations in and around Gaza City

SUNDAY 4 JANUARY

Gaza Strip: Witnesses report clashes around the towns of Beit Hanoun, Beit Lahiya and Jabaliya refugee camp. Witnesses also report Israeli troops in the Zeitoun area, and that the main north-south road is blocked. Israeli troops take control of the site of the former Jewish settlement of Netzarim.Gaza City: Many civilians killed when shells fall near a school and a market, Palestinian sources say.Beit Lahiya: Twelve civilians killed by a tank shell, witnesses say.South Israel: At least 25 short-range rockets fired, mainly on Sderot, Netivot and other towns near the Gaza border. One woman is slightly injured in Sderot.

SATURDAY 3 JANUARY

Gaza Strip: Israeli troops enter northern Gaza, initiating a much-anticipated ground offensive. Hours earlier, Israel fired artillery shells across the border for first time since the offensive began.Gaza City: One of the leaders of Hamas's military wing, Abu Zakaria al-Jamal, was killed in an overnight raid.South Israel: At least 20 Palestinian rockets landed, including in Ashkelon, Ashdod and Sderot. No casualties were reported but a house in Ashkelon was hit.Beit Lahiya: At least 10 Palestinians were killed by an Israeli strike on a mosque, local medical sources said. Earlier, a caretaker was killed in a raid which destroyed large parts of the town's American school.

FRIDAY 2 JANUARY

Khan Younis: Three Palestinian children killed in Israeli air strike.Jabaliya [5]: A mosque described by Israeli security officials as a "terror hub" used to stockpile weapons, was destroyed.Ashkelon: Hamas fired more than 20 rockets into Israel, with some landing in the port town of Ashkelon. No casualties were reported.

THURSDAY 1 JANUARY

Gaza City: The Justice Ministry, Legislative Assembly, Civil Defense Building, Education ministry were hit, as well as at least two money changers' offices and a workshop.Jabaliya [5]: A senior Hamas leader, Nizar Rayyan, was killed in an air strike along with at least nine people, including several members of his family.South Israel: More than 30 rockets fired, including at least one landing in Ashdod and two in Beersheba.

WEDNESDAY 31 DECEMBER

Gaza City: Office of former Prime Minister Ismail Haniya and other Hamas buildings attacked. Israeli aircraft continue to strike tunnels near the border with Egypt.Beersheba: Hit by Hamas rockets for a second day, but no casualties reported.

TUESDAY 30 DECEMBER

Beit Hanoun: Two girls killed in an air strike.Gaza City: At least three buildings in ministry compound hit.Ashdod: Woman killed in rocket attackBeersheba: Attacked by rockets fired from Gaza, the furthest into Israel a Palestinian missile has ever reached.

MONDAY 29 DECEMBER

Nahal Oz: Israeli soldier killed and five others wounded at unspecified military base near a border crossing.Gaza City: Interior Ministry and Islamic University at Tel al-Hawa [1] badly damaged in air strike. Home of Hamas leader Ismail Haniya at Shati refugee camp [3] targeted.Ashkelon: One man killed and several other people injured in rocket attack.

SUNDAY 28 DECEMBER

Jabaliya [5]: Several people killed at a mosque - including five sisters - in air attack at the refugee camp.Yabna refugee camp: Civilian family reported killedRafah: Three brothers reported killed.Khan Younis: Four members of Islamic Jihad and a child reported killed.Ashdod: Palestinian rockets hit the city, the first attack so far north.Deir al-Balah: Palestinians injured, houses and buildings destroyed. Israeli planes strike tunnels running between Egypt and Gaza. Naval vessels targeted at Gaza City port [2] and intelligence building [4] attacked.

SATURDAY 27 DECEMBER

Gaza City, Rafah, Khan Younis: Israel launches a wave of air and missile attacks on targets across Gaza. Some 225 people are killed, according to local medics. Most are policemen within the Hamas militant movement; police chief Tawfik Jaber is among the dead. Women and children also died, according to officials in Gaza.Netivot: One man killed, several injured in Palestinian rocket attack.